1. Field of the Invention
The invention relates to a SiC semiconductor, and in particular to a p-type SiC semiconductor of low resistance.
2. Description of the Related Art
It has been difficult to obtain a p-type SiC semiconductor of low resistance. For example, Published Japanese Translation of PCT application No. 2005-507360 (JP-A-2005-507360) describes the method of obtaining a semi-insulating SiC single crystal of high resistance by employing the combination of deep-energy-level intrinsic defects of high concentration and deep-energy-level acceptors (Ti and B). A method of manufacturing a p-type SiC single crystal by Al-doping, modified Lely method (sublimation method) is described in T. L. Staubinger, et al. Mat. Sci. Forum 389-393 (2002) p. 131. However, the crystal growth with high-concentration Al doping can adversely affect the crystallinity. Japanese Patent Application Publication No. 2008-100890 (JP-A-2008-100890) describes a manufacturing method that is characterized in that the liquid alloy used when a SiC single crystal is grown by liquid-phase growth method, is a Si—Al—M alloy (M is Ti, for example). However, Al and Ti are used to obtain a high-quality SiC single crystal and neither a method of controlling the amounts of these elements that are mixed into the crystal made nor the properties of the crystal are described in this document.
Reasons why it is difficult to manufacture a p-type SiC semiconductor are as follows.
Because in a SiC crystal, holes, which are p-type carriers, are lower in mobility as compared to electrons, which are n-type carriers, it is more difficult to obtain a low-resistance p-type SiC substrate than to obtain a low-resistance n-type SiC substrate.
Al and B are representative acceptors for p-type SiC. However, the ionization energy of Al and B is higher than that of N, which is a donor for n-type SiC. Thus, it is difficult to obtain a low-resistance p-type SiC substrate.
It is possible to use a method of increasing the amount of Al doping in order to reduce resistance. This, however, leads to a reduction in the mobility and the crystallinity of the SiC single crystal can also be reduced.
Thus, a method of independently controlling the amount of Al introduced into the SiC single crystal and the amount of Ti introduced thereinto is needed.
Japanese Patent Application Publication No. 2007-13154 (JP-A-2007-13154), Published Japanese Translation of PCT application No. 2008-505833 (JP-A-2008-505833), WO2004/090969, Japanese Patent Application Publication No. 10-70273 (JP-A-10-70273), and Japanese Patent Application Publication No. 2006-237319 (JP-A-2006-237319) describe that Al and Ti atoms are introduced into a SiC single crystal. However, these documents give no consideration to how to reduce the resistance.